What to Replace Access Database With?

If, for some reason best known the those that came before you, mission critical work was being managed via an Access Database, which itself was really just a front end for what appears to have started out as two massive excel tables, what would you migrate to?

The current database is just a file on network storage and there's about 50 people using it at the same time. It's getting really slow and is constantly needing to be compacted and repaired due to errors. In short a night are to manage. It needs to be replaced. None of the data is normalised and there's loads of duplication, so I personally think it will need to be a clean break rather than a migration. Any suggestions welcome.